COSMODET · Coherent control and molecular dynamics in the era of terahertz

Coherent control and molecular dynamics in the era of terahertz

Molecules in the gas and liquid phases lacking long range order are angularly isotropic, meaning that the molecules are oriented in all space directions with equal probability. Since the interaction of a molecule with light depends on the angle between the polarization of the light and the molecular-frame axis, spectroscopic signals obtained from isotropic samples are inherently averaged over all possible molecular directions, hindering desirable spectroscopic information and prohibiting its association with the frame of the molecule. The COSMODET project was set to develop abilities for controlling the rotational dynamics of molecules in the gas phase by utilizing intense terahertz-frequency (THz) fields and ultrashort near-IR pulses, with the specific aim of achieving molecular ORIENTATION rather than ALIGNMENT. Optically aligned samples maintain their inversion symmetry at all times, a feature emanating from the non-resonant interaction between the short (typically near-IR) pulse and the molecules via their polarizability components rather than their dipole vector. In my lab at Tel-Aviv University, we generate intense THz fields using tilted pulse front optical rectification in LiNbO3. Those single-cycle THz fields interact with polar molecules via their permanent dipole vectors and induce molecular orientation (where the angular distribution of the molecular dipoles is preferential in a specific direction - e.g. toward the positive z-axis direction). Our research focuses on several aspects of the interaction of THz fields with molecules in the gas, liquid and solid phases, including optically induced air plasma. Our work under the COSMODET project have yielded several achievements - some were suggested in the original proposal and others arose throughout the work: We studied the rotational responses of asymmetric molecules to THz and optical pulses and have shown that these two kinds of interaction (rotationally resonant and non-resonant respectively) provide two distinct rotational handles over asymmetric molecules. The experimental work is supported by theoretical calculations using the Random Phase Wave Function method that was implemented for this task (an important theoretical advancement for calculating the rotational responses of asymmetric molecules at ambient temperature - where exact calculation methods are practically not applicable). In a long series of rotational dynamics measurements and decay analysis we have found that field-free oriented molecules decay with a faster rate compared to field-free aligned molecules. The effect was termed 'coherent radiative decay' and suggests that the free-induction signal emitted from coherently rotating molecules upon their periodic orientation results in a decay phenomenon that was discarded before in the field of molecular dynamics. While working on decay phenomena in rotational dynamics we delved into rotational-echo spectroscopy - a well established technique in Magnetic Resonance, electronic and vibrational spectroscopy that have emerged into rotational spectroscopy only recently. On this front, we have demonstrated the ability to rephase the centrifugal distortion of rotational-echo signals and are working to utilize this technique to decipher between dephasing and decoherence. Our work on rotational decay have yielded another, previously unexpected, decay phenomenon that is at the heart of my current research and emerged from our work on the COSMODET project. Transfer of Knowledge ------------------------------ In addition to the research activities, I am taking an active role as a faculty member in the School of Chemistry in Tel Aviv University. I am teaching the spectroscopy course for third year undergraduate students, and two physical chemistry laboratory courses (I + II). These courses, attended by all of the students in chemistry, provide an excellent platform to transfer knowledge on my field of expertise and attract undergraduate students toward spectroscopy and light-matter interactions. In the last two years I've taken 3 undergraduate students for 3 months research projects. While these research projects require significant investment of my time and effort, they are probably the best platform for attracting students to ultrafast spectroscopy and optics and to build some experience in hands-on optics. In addition to the courses, I am taking active part in additional activities aimed at transferring knowledge specifically in my field of research. These include two-hour lectures on THz and optical induced rotational dynamics given to excellent chemistry students within a two-week summer workshop that I am guiding together with another faculty member. These lecture are also given in the 'Ofakim bechimia' (horizons in chemical research) attended by our undergraduate students, and to physics students (undergraduate + graduate) in the course 'chemistry for physicists'. I also gave a talk at the OSA student chapter activity at TAU attended by graduate students from the schools of engineering, physics and chemistry. Since I joined Tel Aviv University I was invited to give seminars on my research work in chemistry and physics departments in several Israeli universities, Israeli research institutes and conferences. Perhaps the most outreaching project that I am involved in is "Niflaot Hachimia" (translated to Wonders of Chemistry) - where 3-4 times every year I meet with students in their 11th or 12th grade at school (age 16-18), and give them a 1hour talk titled "when light and matter meet" followed by a lab tour. This project is one of the initiative at the school of chemistry at TAU to try and interest the next generation of potential scientists. Through this project we meet children from all the places and backgrounds in Israel (also all religions). Whether it is efficient and will it bring more students to study chemistry and physics in the future? we certainly hope so! but will know for sure in a few years...

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